Silver-containing direct thermographic and photothermographic imaging materials (that is, thermally developable imaging materials) that are imaged and/or developed using heat and without liquid processing have been known in the art for many years.
Silver-containing direct thermographic imaging materials are non-photosensitive materials that are used in a recording process wherein images are generated by the direct application of thermal energy and in the absence of a processing solvent. These materials generally comprise a support having disposed thereon (a) a relatively or completely non-photosensitive source of reducible silver ions, (b) a reducing composition (acting as a black-and-white silver developer) for the reducible silver ions, and (c) a suitable binder. Thermographic materials are sometimes called “direct thermal” materials in the art because they are directly imaged by a source of thermal energy without any transfer of the image or image-forming materials to another element (such as in thermal dye transfer).
In a typical thermographic construction, the image-forming thermographic layers comprise non-photosensitive reducible silver salts of long chain fatty acids. A preferred non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms, such as behenic acid or mixtures of acids of similar molecular weight. At elevated temperatures, the silver of the silver carboxylate is reduced by a reducing agent for silver ion (also known as a developer), whereby elemental silver is formed. Preferred reducing agents include methyl gallate, hydroquinone, substituted-hydroquinones, hindered phenols, catechols, pyrogallol, ascorbic acid, and ascorbic acid derivatives.
Some thermographic constructions are imaged by contacting them with the thermal head of a thermographic recording apparatus such as a thermal print-head of a thermal printer or thermal facsimile. In such constructions, an anti-stick layer is coated on top of the imaging layer to prevent sticking of the thermographic construction to the thermal head of the apparatus utilized. The resulting thermographic construction is then heated imagewise to an elevated temperature, typically in the range of from about 60 to about 225° C., resulting in the formation of a black-and-white image of silver.
Silver-containing photothermographic imaging materials (that is, photosensitive thermally developable imaging materials) that are imaged with actinic radiation and then developed using heat and without liquid processing, have also been known in the art for many years. Such materials are used in a recording process wherein an image is formed by imagewise exposure of the photothermographic material to specific electromagnetic radiation (for example, X-radiation, or ultraviolet, visible, or infrared radiation) and developed by the use of thermal energy. These materials, also known as “dry silver” materials, generally comprise a support having coated thereon: (a) a photocatalyst (that is, a photosensitive compound such as silver halide) that upon such exposure provides a latent image in exposed grains that are capable of acting as a catalyst for the subsequent formation of a silver image in a development step, (b) a relatively or completely non-photosensitive source of reducible silver ions, (c) a reducing composition (usually including a developer) for the reducible silver ions, and (d) a binder. The latent image is then developed by application of thermal energy.
In photothermographic materials, exposure of the photosensitive silver halide to light produces small clusters containing silver atoms (Ag0)n. The imagewise distribution of these clusters, known in the art as a latent image, is generally not visible by ordinary means. Thus, the photosensitive material must be further developed to produce a visible image. This is accomplished by the reduction of silver ions that are in catalytic proximity to silver halide grains bearing the silver-containing clusters of the latent image. This produces a black-and-white image. The non-photosensitive silver source is catalytically reduced to form the visible black-and-white negative image of silver while much of the silver halide, generally, remains as silver halide and is not reduced.
In photothermographic materials, the reducing agent for the reducible silver ions, often referred to as a “developer”, may be any compound that, in the presence of the latent image, can reduce silver ion to metallic silver and is preferably of relatively low activity until it is heated to a temperature sufficient to cause the reaction. A wide variety of classes of compounds have been disclosed in the literature that function as developers for photothermographic materials. Upon heating, and at elevated temperatures, the reducible silver ions are reduced by the reducing agent. This reaction occurs preferentially in the regions surrounding the latent image. This reaction produces a negative image of metallic silver having a color that ranges from yellow to deep black depending upon the presence of toning agents and other components in the photothermographic imaging layer(s).
Differences Between Photothermography and Photography
The imaging arts have long recognized that the field of photothermography is clearly distinct from that of photography. Photothermographic materials differ significantly from conventional silver halide photographic materials that require processing with aqueous processing solutions.
In photothermographic imaging materials, a visible image is created in the absence of a processing solvent by heat as a result of the reaction of a reducing agent incorporated within the material. Heating at 50° C. or more is essential for this dry development. In contrast, conventional photographic imaging materials require processing in aqueous processing baths at more moderate temperatures (from 30° C. to 50° C.) to provide a visible image.
In photothermographic materials, only a small amount of silver halide is used to capture light and a non-photosensitive source of reducible silver ions (for example, a silver carboxylate or a silver benzotriazole) is used to generate the visible image using thermal development. Thus, the imaged photosensitive silver halide serves as a catalyst for the physical development process involving the non-photosensitive source of reducible silver ions and the incorporated reducing agent. In contrast, conventional wet-processed, black-and-white photographic materials use only one form of silver (that is, silver halide) that, upon chemical development, is itself at least partially converted into the silver image, or that upon physical development requires addition of an external silver source (or other reducible metal ions that form black images upon reduction to the corresponding metal). Thus, photothermographic materials require an amount of silver halide per unit area that is only a fraction of that used in conventional wet-processed photographic materials.
In photothermographic materials, all of the “chemistry” for imaging is incorporated within the material itself. For example, such materials include a developer (that is, a reducing agent for the reducible silver ions) while conventional photographic materials usually do not. The incorporation of the reducing agent into photothermographic materials can lead to increased formation of various types of “fog” or other undesirable sensitometric side effects. Therefore, much effort has gone into the preparation and manufacture of photothermographic materials to minimize these problems.
Moreover, in photothermographic materials, the unexposed silver halide generally remains intact after development and the material must be stabilized against further imaging and development. In contrast, silver halide is removed from conventional photographic materials after solution development to prevent further imaging (that is in the aqueous fixing step).
Because photothermographic materials require dry thermal processing, they present distinctly different problems and require different materials in manufacture and use, compared to conventional, wet-processed silver halide photographic materials. Additives that have one effect in conventional silver halide photographic materials may behave quite differently when incorporated in photothermographic materials where the underlying chemistry is significantly more complex. The incorporation of such additives as, for example, stabilizers, antifoggants, speed enhancers, supersensitizers, and spectral and chemical sensitizers in conventional photographic materials is not predictive of whether such additives will prove beneficial or detrimental in photothermographic materials. For example, it is not uncommon for a photographic antifoggant useful in conventional photographic materials to cause various types of fog when incorporated into photothermographic materials, or for supersensitizers that are effective in photographic materials to be inactive in photothermographic materials.
These and other distinctions between photothermographic and photographic materials are described in Unconventional Imaging Processes, E. Brinckman et al. (Eds.), The Focal Press, London and New York, 1978, pp. 74-75, in D. H. Klosterboer, Imaging Processes and Materials, (Neblette's Eighth Edition), J. Sturge, V. Walworth, and A. Shepp, Eds., Van Nostrand-Reinhold, New York, 1989, Chapter 9, pp. 279-291, in Zou et al., J. Imaging Sci. Technol. 1996, 40, pp. 94-103, and in M. R. V Sahyun, J. Imaging Sci. Technol. 1998, 42, 23.
Problem to be Solved
Photothermographic and thermographic materials have been designed and commercialized with one or more backside (non-imaging side) layers for a number of years. Such layers are designed to have various functional properties including conductive (or antistatic), antihalation, protective, and improved machine feeding and transport properties. Efforts have been continuing to provide backside layers with improved functional properties as well as to reduce manufacturing and component costs.
Many of the chemicals used to make supports or supported layers in thermally developable materials have electrically insulating properties, and electrostatic charges frequently build up on the materials during manufacture, packaging, and use. The accumulated charges can cause various problems. For example, in photothermographic materials containing photosensitive silver halides, accumulated electrostatic charge can generate light to which the silver halides are sensitive. This may result in imaging defects that are a particular problem where the images are used for medical diagnosis.
Build-up of electrostatic charge can also cause sheets of thermally processable materials to stick together causing misfeeds and jamming within processing equipment. Additionally, accumulated electrostatic charge can attract dust or other particulate matter to the materials, thereby requiring more cleaning to insure rapid transport through the processing equipment and quality imaging.
Build-up of electrostatic charge also makes handling of developed sheets of imaged material more difficult. For example, radiologists desire a static free sheet for viewing on light boxes. This problem can be particularly severe when reviewing an imaged film that has been stored for a long period of time because many antistatic materials loose their effectiveness over time.
In general, build-up of electrostatic charge is related to surface resistivity (measured in log ohm/sq) and rate of charge decay. While electrostatic charge control agents (also known as “antistatic agents” or “antistats”) can be included in any layer of an imaging material, the accumulation of electrostatic charge can be prevented by reducing the surface resistivity or by lowering the charge level. These results can most often be achieved by including charge control agents in surface layers such as protective overcoats or in buried layers. In thermally processable materials, charge control agents may be used in backing layers that are on the opposite side of the support as the imaging layers.
As noted above, there are many conductive compounds described for use in imaging materials. However, it is generally impossible to merely choose a compound from the literature and include it in an antistatic composition for use in specific types of imaging materials. There are so many parameters and properties that are affected by each coating component and while the desired conductivity may be achieved, other properties may be adversely affected. A careful balancing act is required by the industrial researcher to find the best conductive compounds that achieve as many desired properties as possible. Optimization to achieve all desired properties is usually more than a routine task.
For example, to be useful in a “buried” conductive backside layer in a hydrophobic binder coated from non-polar organic solvents, a conductive compound generally must be compatible with the hydrophobic binder and have no detrimental effects upon any components in any overcoat layers, have only minimal migration into any overcoat or adjacent layers, generate minimal haze on the backside, and provide a log resistivity of less than 1012 ohm/sq that is relatively invariant with changes in humidity. In addition, the conductive material should not disrupt adhesion between the polymeric support and any overcoat or undercoat layers. It should readily dissipate charge and have a static decay time of 100 seconds or less (preferably 25 seconds or less).
Preferably, the conductive materials should not cause surface irregularities such as blush, smearing, or coating voids (known as “de-wetting”) on any overcoat layers and should not react with the polymer binders or other materials in other backside layers of the thermally developable materials during storage, imaging, development, or post-development storage.
A wide variety of organic and inorganic charge control agents have been devised and used for electrostatic charge control. Metal oxides are described in conductive layers in U.S. Pat. Nos. 5,340,676 (Anderson et al.), 5,368,995 (Christian et al.), 5,457,013 (Christian et al.), 6,464,413 (Oyamada), and 5,731,119 (Eichorst et al.).
U.S. Pat. No. 6,355,405 (Ludemann et al.) describes thermally developable materials that include very thin adhesion-promoting layers on either side of the support. These adhesion-promoting layers are also known as “carrier” layers. U.S. Pat. No. 6,689,546 (LaBelle et al.) describes thermally developable materials that contain a buried conductive backside “carrier” layer comprising non-acicular metal antimonate particles.
Various fluorochemicals have been used for the purpose of reducing the generation of surface electrostatic charge as described, for example, in U.S. Pat. Nos. 5,674,671 (Brandon et al.), 6,171,707 (Gomez et al.), 6,287,754 (Melpolder et al.), and 6,699,648 (Sakizadeh et al.). Polyoxyalkyleneamines that are useful for making such fluorinated organic salts include JEFFAMINE® compounds (currently available from Huntsman Corp.). One such electrostatic charge control agent is the perfluorooctylsulfonyl (PFOS) salt of a polyoxyalkyleneamine described as Compound 1 in U.S. Pat. No. 4,975,363 (Cavallo et al.).
U.S. Pat. No. 6,811,724 and U.S. Patent Application Publication 2005/0006629 (Majumdar et al.) describe the use of a combination of a polyethylene ether glycol and lithium nitrate in an antistatic layer.
Despite the considerable research and knowledge in the art relating to various conductive polymers, metal oxide particles, fluorochemicals and other materials in conductive compositions and imaging materials, there remains a need for additional conductive materials for use layers on the backside of thermally developable materials that are low in cost, can be coated from organic solvents, and meet many if not all of the general or specific requirements described above for conductive or antistatic layers.